1. Field of the Invention
This invention relates to a viscous fluid coupling, more particularly a temperature sensitive viscous fluid coupling which can control the transmission of the output torque in three steps in accordance with temperature, and can be used as a cooling fan for automobile engines.
2. Description of the Prior Art
A known prior art related to this invention is described in the Japanese Laid-open Patent Publication No. 69326/1980 (Toku-kai-sho No. 55-69326). The device disclosed in the above publication has the structure as illustrated in the attached FIG. 5 and FIG. 6. Casing 53 is supported to be rotatable on rotating shaft 51 driven by an engine through bearing 52. Cover 54 is fixed on casing 53 so as to form internal space together with said casing 53, and rotor 55 is connected to rotating shaft 51. The internal space is separated by partition 57 fixed to cover 54 into working chamber 58 that accommodates rotor 55 and storage chamber 59 that stores viscous fluid. The first torque transmitting surface 56 comprising labyrinth grooves is formed on the opposing surfaces of rotor 55 and casing 53, whereas the second torque transmitting surface 61 comprising labyrinth grooves is formed on the opposing surfaces of rotor 55 and partition 57. The first return hole 62 and second return hole 60 that reflux viscous fluid from storage chamber 59 to working chamber 58 are formed in partition 57 (the first return hole being formed on the outer periphery side in the radial direction and the second return hole on the inner periphery side), spiral bimetal 65 that operates by detecting the temperature of the air that passes through the radiator is mounted on the front of cover 54, and the opening and closing of the both return holes 62 and 60 are controlled by valve plate 67 that is connected to said bimetal 65 and that moves around on partition 57. That is to say, valve plate 67 is held in position a where both return holes 62 and 60 close at low temperature, is held in position b where only the first return hole 62 formed on the outer peripheral side in the radial direction opens when the temperature rises and reaches the first specified temperature T1, and is held in position c where the second return hole formed on the inner periphery side in the radial direction also opens when the temperature rises further and reaches the second specified temperature T2 (where T1&lt;T2), as illustrated in FIG. 6. Since the torque transmission from rotating shaft (input member) 51 to casing 53 and cover 54 (output member) is controlled in three steps in this manner, the rotational speed of the cooling fan installed on the output member is controlled in three steps: i.e. low speed rotation (OFF state) at a low temperature lower than the first specified temperature T1, medium speed rotation (MIDDLE state) at a medium temperature between the first specified temperature T1 and the second specified temperature, and high speed rotation (ON state) at a high temperature higher than the second specified temperature T2.
As mentioned above, the conventional device detects the temperature of the air that passes through the radiator by spiral bimetal to control the rotational speed of the cooling fan installed on the output member in three steps of OFF, MIDDLE and ON. Therefore, sometimes the fan rotates at high speed even when the temperature of the engine-cooling water is low. For instance, when the vehicle is running on urban streets, and the temperature of air passing through the radiator is low with no temperature rise of the water in the engine (namely, the temperature corresponding to the OFF or MIDDLE state), the fan should rotate at a low or middle speed. Nevertheless, under such condition, the fan tends to rotate at a high speed in the "ON" state, because the volume of air passing through the radiator is small, the velocity of the passing air is low, and air blown to the bimetal mounted on the front of the output member is weak.